EFFECTS OF METAL IONS ON OXYGEN DELIGNIFICATION

The oxygen delignification process involves a gas, liquid and a solid phase. The solid phase is a heterogeneous composite of two intermixed organic polymer phases /10/.

Given these conditions, it is obvious that much of the knowledge on the process is based on the empirical data. For example, the effect of metal ions in oxygen delignification is still not satisfactorily understood despite several studies /10/.

Generally, magnesium compounds are considered to be beneficial for oxygen delignification and are the only accepted reagents for improving the selectivity of the

process. Several theories have been proposed to explain the beneficial effects of magnesium:

1) The stabilization of intermediate radical species by magnesium /10/

2) The coprecipitation of transition metal ions with magnesium hydroxide which should stabilize hydrogen peroxide against decomposition to give hydroxyl radicals and achieve the redox stabilization of Mn²⁺ /8/.

3) The formation of Mg-cellulose transient complexes which protects cellulose against attacks from hydroxyl radicals /8, 10/.

4) The association of superoxide with the Mg(OH)2 colloid may catalyse the proton-dependent dismutation of superoxide; i.e. the Mg(OH)2 colloid mimics superoxide dismutase /8/.

Manganese, which is one of the transition metals, is considered to be benign or even beneficial, while iron and copper have been found to be detrimental to pulp strength, because they tend to react with cellulose in undesirable depolymerization reactions.

Calcium has been found to be deterious to the pulp viscosity, either because it displaces beneficial magnesium or because it is inherently detrimental to cellulose during oxygen delignification /11/. Decreasing the calcium level of pulp while retaining beneficial magnesium does not appear to be an easily attainable goal. It may be that the most practical approach that can be taken to limit the amount of calcium present in brown stock is to ensure that the calcium levels in the white liquor are kept as low as possible /11/.

3 FIBER DAMAGE

3.1 THE NATURE OF FIBER DAMAGE

Various types of stress can induce deformations in wood fibers. The fiber deformations already present in a living tree may be enlarged by the mechanical and chemical treatment during pulp processing. Also, new deformations may develop, which leads to reductions in the fiber strength /12/. The wood species, growth conditions and fiber properties all affect the susceptibility of fibers to damage during pulp processing. For example, thick-walled fibers are more susceptible to fiber

damage due to mechanical processing than are thin-walled fibers, which can bend elastically /12, 13/.

Previous studies into fiber damages in the brown stock line during kraft pulp processing have shown that no single process step can be identified to be the source of damage. Damages in the fiber form occur in many steps; for example, in the blow valve, blow tank pump and valve, screening and the oxygen stage /14/. It has also been confirmed that the discharge of cooked chips from the digester results in numerous irreversible fiber damages /12/.

The true nature of fiber damages is not yet completely clear /15/. All fiber properties may vary, and many of them correlate vaguely with the loss of fiber strength measured in terms of the wet zero-span, tensile and tear strength. The strongest correlations with the loss of fiber strength are evident with fiber deformation indexes such as curliness and the fiber dislocation index /15/. In addition to fiber curl and dislocations, twists, kinks, bends, holes in the cell wall, swelling, microcompression, collapsed cell walls and cut fibers are other examples of fiber form defects /16/.

Fiber deformations decrease the ability of fibers to transmit load, which is seen as decrease in paper strength properties; tensile strength, tensile stiffness and burst strength /17/. The strength of pulps consists of at least three components; single fiber strength, fiber-fiber bond strength per unit area and the total fiber-fiber bonded area /18/. Individual fiber strength properties are a direct result of the initial fiber strength in the original woodchips, and of the chemical and mechanical treatment in the mill.

Once the pulp has left the process, it is not possible to improve an individual fiber strength /18/. Furthermore, the detection of single fiber strength losses using conventional test methods is difficult. For example, in zero-span measurement, the fiber strength has to be calculated based on a number of assumptions, e.g. the fiber distribution, fiber network properties etc. /18/. The other two components, the bond strength and bonded area are considered to be less influenced by the process conditions, and these components can be influenced after the fibers have left the process /18/.

3.1.1 Dislocations

Dislocations are already common in native softwood tracheids. Scientists suggest that dislocations are a product of the growth or wind stress of a tree and that dislocations and zones of dislocations can give rise to microcompressions under drying stress /19/.

During mechanical treatment, i.e. chipping, pulping and pulp processing, dislocations arise in pulp fibers due to the compressive stresses acting on the fiber wall /20/.

Fiber failure starts from microscopic damage in the fibrils. The axial compression of fibers results in dislocations and misalignments. After this, the fiber does not carry as much load as does the undamaged fiber /15/. The fiber wall becomes weaker, and this weakening cannot be explained by viscosity or chemical composition, nor by the 3-D shape of the fibers /15/. Dislocations make fibers more flexible and sensitive to chemical attack with an accompanying improvement in the binding capacity /12/.

Tracheids tend to swell, bend and rupture at the sites of dislocations, thus resulting in the decreased strength capacities of the pulp. Figure 3 illustrates a fiber that contains dislocations that result from longitudinal compression.

Figure 3. Dislocation areas across the width of a spruce fiber (scale bar = 40 mm) /12/.

Dislocations that result from the longitudinal compression of wood can be seen, according to W. Robinson, as bright, linear regions in the fiber wall when viewed under polarized light /21/. The observed white lines presumably relate to the structural changes initiated in the S1 layer /12/. It is likely that the differences in microfibril

orientation in the S1 layer govern the direction of the dislocations. Increased longitudinal compression can eventually lead to the separation of the S1 and S2 layers and, finally, to gross buckling, severe folding and the complete failure of fibers /12/.

According to Forgacs, the size of dislocations varies a lot, and their length may be 5-30 mm in fibers the width of which is around 30 mm /22/. According to Nyholm et al., the average length of dislocations (microcompressions) is 20-120 mm /12/. Smaller dislocations, often called slip planes, are approximately 1 mm in width, and can be seen as both single and double minute compression failures /12/. Single compressions are considered to be single folds that are associated with kinks in the fiber wall.

Double compressions are found with the angle opening either towards the lumen or outer cell wall, which is caused by a thickening in the direction in which the angle opens. Dislocations occur with equal frequency in both early- and latewood tracheids but are more pronounced in latewood tracheids, probably because of their thicker cell walls /22/. However, it has been found that beaten earlywood tends to exhibit more dislocations than latewood fibers /12/.

Most scientists, who study fiber dislocations, believe that there is a correlation between dislocations and ray crossings /12/. When studying spruce fibers, researchers found that weak planes and most discontinuities occur preferentially near ray pit fields /12/. The edges of ray cross-field pits are areas where wood tends to fold and fracture under tension and compression /12/. When applying compressive strength on wet wood, dislocations appear at sites of fiber/ray cell contact in latewood rows.

Normally, rays have a major role in radial shear parallel to the grain, and step-wise failure results due to the area of weakness they represent /12/. Areas of weakness are also found close to the tracheid ends, where the cells often deviate from their vertical alignment and are heavily pitted /12/.

3.1.2 Fiber Curl and Stretch

Curly fibers are a problem as they tend to lead to weaker anisotropy in paper properties than straight fibers /23/. Fiber interactions and flocculation resist the

rotation of fibers, and fibers may therefore bend during drainage /23/. In a low-density network, fiber curl and wrinkles reduce the critical buckling stress /1/. Fiber curl is especially undesired in pulp intended for carton board production, as it tends to decrease the elastic modulus of the pulp and thus increase the stretch of carton board /2/.

Fiber curl occurs when the fiber microstructure changes, i.e. inner fibrillation unfolds.

This happens especially in alkaline conditions. When the alkali is removed, the reactive sites of the fibrils bond with each other. The bonding, however, does not occur at original sites for many reasons; for example, lignin or hemicellulose has been removed from the fiber leaving an open reactive site, or the mechanical forces have twisted the fiber so that sites physically close to each other are susceptible to bonding.

The factors that affect this bonding are largely unclear, as is the time in which this reattachment happens /13/.

Pulp curl seems to be yield-dependent, i.e. the lower the yield, the curlier are the fibers /17/. Sundquist and Tikka observed that fiber curling occurs mainly during brown stock processing and can be up to 130-150% /13, 14/. Fiber curliness also tends to vary a lot in pulp samples from different mills depending on the process /13/.

However, it is common for all pulp samples that the curliness of the final pulp tends to reach a “minimum” curliness level despite different processing conditions /13/.

The degree of fiber curl can be changed through beating the pulp to different degrees depending on beating conditions /17/. Laboratory-scale PFI beating produces straight and evenly treated fibers; however, according to Mohlin et al., mill-scale beating has a weaker fiber straightening effect /17/. Fiber deformations may also “heal” during the drying process in paper manufacturing /24/. The straightening of fibers in macroscopic drying increases the elastic modulus, and the fiber becomes better aligned. In some paper grades, fiber curl can even be beneficial by adding more porosity and bulk to the paper sheet /2/.

3.2 SWELLING OF FIBERS

The water intake and swelling of pulp fibers are important factors which have a strong influence on the consolidation and bonding of the fibers in the web and are, thus, important for the final quality of paper products /25/. Water is held by the fibers in many different ways; in the amorphous polymer parts of the fiber cell wall, in cracks and pores in the lumen and on the surfaces of the fiber, as well as by the fibrillar surface gel /25/.

The cell walls of dry fibers do not contain any pores. During the growth and the formation of the cell wall, the removal of water brings the structural elements together, and consequently, the cell wall shrinks. Subsequently, cell wall pores develop in the fiber during swelling, caused by water, which pushes the fibrils apart, allowing the structure to return, at least partly, to its original state /26/. The pores in the swollen, delignified cell wall appear to be rather uniform with an average size of around 100 nm in diameter, with the exception of possible micropores of an equivalent cylindrical size of around 2 nm /26, 27/. Figure 4 shows a picture of a swollen fiber surface.

Fibers immersed in water swell until equilibrium is established between the water in the fibers and the water in the surrounding solution, i.e. until the chemical potential of the water is the same everywhere /25/. According to Scallan et al., swelling results from the osmotic pressure generated within the fiber wall when the counter-ions of the acidic groups are exchanged from hydrogen to sodium form /27/. The degree of swelling depends on the temperature, ionic strength, chemical composition and internal fibrillation of the fibers, as well as on the mechanical restraints to the swelling of the wood fiber material /25/.

Figure 4. SEM micrographs of a freeze-dried, swollen unbeaten kraft fiber surface, illustrating the separation of the fibrils and the formation of openings (bar 100 nm) /26/.

In the beginning of delignification, the amount of bound water increases, which is a reflection of an increase in the amorphous (cellulose and hemicelluloses) carbohydrate content /25/. The presence of water converts the ligno-hemicellulose (located in the pores) to a micro-porous gel, where lignin acts as a cross-linking agent within the wall and hemicellulose acts as a coupling agent between lignin and cellulose /27/. Hemicelluloses promote fiber swelling, and lignin inhibits it /24/. In the later state of delignification, when the hemicelluloses have been removed, the remaining wood polymers are less water-absorbing due to their higher content of crystalline cellulose /25/. The pore volume also increases at first, due to the cavities developed when lignin is removed /28/. However, extensive delignification leads to a collapse of these cavities, lowering the amount of pore water /25/. It is probable that with progressive lignin removal, small pores develop into larger ones; also, the cell wall may collapse at low yields /25/. Furthermore, fiber damages lead to a decrease in the ability to retain water /29/.

Hartler suggested that swelling in a wood fiber can only take place towards the lumen and is, therefore, the only possible direction of expansion of the fiber cell wall /30/.

According to Scallan et al., swelling occurs principally in a direction transverse to the microfibrils. In addition, as swelling progresses, the elastic nature of the cell wall resists the expansion until equilibrium is achieved /27/. The modification of the ligno-hemicellulose gel, for example during pulping, reduces the cross-linking effect of

ligno-hemicellulose, leading to a drop in the elastic modulus /27/. Thus, low-yield pulping leads to the collapse of pulp fibers /27/. The elastic modulus can, however, be increased through drying-and-rewetting, due to the formation of hydrogen bonds between the microfibrils /27/.

The swelling of fibers can cause them to straighten /24/. Swelling also yields more flexible fibers, and the increased fiber flexibility promotes conformability, thus allowing the fibers to form more fiber-fiber contacts and achieve a better web strength /31/. Certain chemical groups - such as carboxylic groups - dissociate in water, promoting fiber swelling and contribute to the improved flexibility and conformability of the fiber wall /24/. High contents of carboxylic acid groups on fiber surfaces may also increase the inter-fiber bonding strength /32/.

3.3 ANALYSIS METHODS OF FIBER DAMAGE

The deterioration of the pulp strength tends to vary in different mills and processes.

However, the reduction of the tensile and tear strength and the Zero Span, which describes the deterioration of the fiber strength in the brown stock line, are common to every process. The most important task of fiber deformation analyses is to evaluate the effect of fiber deformation on the ability of the fibers to take up load and to transmit load /17/. When studying fiber damages most commonly used analyses are pulp viscosity, tensile and tear strength measurements, zero-span, wet zero-span, fiber curl and stretch. A more novel method of testing individual fiber strengths is the Single Fiber Fragmentation Technique (SFF), which enables the evaluation of the effect of an individual fiber strength on the loss in pulp strength /18/.

Fiber curl and dislocations can be determined via microscopic analyses, and nowadays, also automatically (Pulp Expert). The strength deterioration of individual fibers can be measured using wet zero span, however the local defects found in fibers probably have no effect on the zero-span measurement /12/. According to Mohlin et al., wet zero span is a good way of measuring the ability of fibers to carry load, as it is not affected by fiber-fiber bonds /17/. In fact, in their studies, they observed a good correlation between wet zero-span values and curl and kink indexes /17/. However,

although a change in the visual deformation of fibers is related to changes in their ability to transmit load, their shape (curl and kink indexes) cannot be used as a general measure when different pulps are to be compared /17/.

The curl and kink indexes are defined as follows:

Curl index = (fiber contour length / longest dimension)-1

Kink index = (N10-20+2N21-45+3N46-90+4N91-180) / total fiber length,

Where N is the number of kinks with an angle in the interval indicated by the subscript /17/.

Fiber dislocations and microcompressions can be determined using staining analyses.

Poorly oriented and loose cellulose regions of cell walls stain more strongly than do the surrounding intact walls. For example, Congo red have be used to indicate cell wall discontinuities i.e. dislocations /12/.

Tracheids are susceptible to chemical attacks at sites of dislocations. The separation of cell wall microfibrils or bundles or the change of the direction of microfibrils lead to the breaking of hydrogen bonds, which results in a higher accessibility of cellulose to chemical hydrolysis /12/. The influence of acids, enzymes and other agents is related to the extent of the structural changes, with larger deformations increasing the accessibility of cell wall elements /12/. This feature can be used in studying fiber dislocations with the help of cellulase treatment. Cellulases are able to attack structurally irregular zones in the fiber wall, and penetrate the ruptured S1 layer. This results in localized sites of degradation and, eventually, in the more rapid degradation of the fiber /12/.

3.4 POSSIBLE CAUSES FOR FIBER DAMAGE

The development of pulping technology has lead to dramatic changes in digester designs and volumes /33/. A major step towards modern digester technology was the shift from moving reactors to stationary ones, which enabled greater capacities and higher levels of production /15/. Serving as the first and main delignification and

defibration step, the digester is in a crucial position in the control of the pulp strength /15/.

So far, apart from already known detrimental effects of prolonged delignification, no single process has been clearly found to be the source for fiber damage in the downstream brown stock line. Changes in fiber properties occur in many steps and are usually caused by several factors. Pulping chemistry alone cannot explain the loss of mill pulp strength. Generally it can be said that treatments, which require high shear forces, high pH levels and temperatures, high consistencies and amounts of chemicals, and which contain contaminants or other impurities, result in fiber damages /2/. For example, Tikka et al. observed that dissolved solids and ion concentration play important roles in the genesis of the strength losses /15/. Fiber damages can occur in the fiber line between the digester and the oxygen stage if the temperature or pH is too high, especially when the pulp fibers are subjected to simultaneous intensive mechanical action /18/. Mechanical treatment can lead to increased curl and micro-compressions, which can decrease the sheet tensile properties /18/.

After the introduction of medium consistency pumping and high shear force fluidization, some questions were raised about their effects on the quality of the pulp /15/. These questions have mostly remained unanswered, because true chemical and physical mill conditions have not been or could not be used in laboratory-scale studies

After the introduction of medium consistency pumping and high shear force fluidization, some questions were raised about their effects on the quality of the pulp /15/. These questions have mostly remained unanswered, because true chemical and physical mill conditions have not been or could not be used in laboratory-scale studies